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| United States Patent |
5,092,977
|
|
Bachot
, et al.
|
March 3, 1992
|
Microporous asbestos diaphragms/cathodes for electrolytic cells
Abstract
Microporous diaphragms adapted for wet-consolidation with composite
cathodes for use in electrolytic cells, e.g., chlorine/soda electrolysis
cells, comprise a sintered, fluoropolymer microconsolidated asbestos-based
microporous fibrous sheet material, such sheet material including from 3%
to 35% by weight of fluoropolymer binder, from 1% to 50% by weight of a
uniformly distributed gel of an oxohydroxide of at least one metal of
Groups IVA, IVB, VB and VIB of the Periodic Table or of the lanthanide or
actinide series thereof, and from 20% to 95% by weight of fibers, at least
1% by weight of such fibers being asbestos fibers.
| Inventors:
|
Bachot; Jean (Bourg la Reine, FR);
Stutzmann; Pascal (Paris, FR);
Perineau; Jean-Maurice (Claix, FR)
|
| Assignee:
|
Rhone-Poulenc Chimie (Courbevoie, FR)
|
| Appl. No.:
|
565344 |
| Filed:
|
August 10, 1990 |
Foreign Application Priority Data
| Current U.S. Class: |
204/252; 204/282; 204/283; 204/296 |
| Intern'l Class: |
C25B 013/08 |
| Field of Search: |
204/295,296,283,252,282
427/77,245,58
|
References Cited
U.S. Patent Documents
| 3980613 | Sep., 1976 | Bachot et al. | 264/45.
|
| 4105516 | Aug., 1978 | Martinsons et al. | 204/129.
|
| 4354900 | Oct., 1982 | Bailey et al. | 162/106.
|
| 4410411 | Oct., 1983 | Fenn, III et al. | 204/283.
|
| 4447566 | May., 1984 | Hruska et al. | 523/221.
|
| 4563260 | Jan., 1986 | Hruska et al. | 204/283.
|
| 4665120 | May., 1987 | Hruska et al. | 524/452.
|
| 4701250 | Oct., 1987 | Fenn, III et al. | 204/283.
|
| 4720334 | Jan., 1988 | DuBois et al. | 204/296.
|
| 4743349 | May., 1988 | Bachot et al. | 204/242.
|
| 4775551 | Oct., 1988 | Bachot et al. | 427/58.
|
| 4810345 | Mar., 1989 | Schulz et al. | 204/295.
|
| 4939028 | Jul., 1990 | Bachot et al. | 428/311.
|
| 4940524 | Jul., 1990 | Perineau et al. | 204/242.
|
| 5023127 | Jun., 1991 | Bachot et al. | 428/137.
|
| Foreign Patent Documents |
| 0132425 | Jan., 1985 | EP.
| |
| 0214066 | Mar., 1987 | EP.
| |
| 0296076 | Dec., 1988 | EP.
| |
| 2213805 | Aug., 1974 | FR.
| |
| 2229739 | Dec., 1974 | FR.
| |
Primary Examiner: Niebling; John
Assistant Examiner: Gorgos; Kathryn
Attorney, Agent or Firm: Burns, Doane, Swecker & Mathis
Claims
What is claimed is:
1. A microporous diaphragm which comprises a sintered, fluoropolymer
microconsolidated asbestos-based microporous fibrous sheet material, said
sheet material comprising from 3% to 35% by weight of fluoropolymer
binder, from 1% to 50% by weight of a uniformly distributed gel of an
oxohydroxide of at least one metal of Groups IVA, IVB, VB and VIB of the
Periodic Table or of the lanthanide or actinide series thereof, and from
20% to 95% by weight of fibers, at least 1% by weight of said fibers being
asbestos fibers.
2. The microporous diaphragm as defined by claim 1, at least 5% by weight
of said fibers being asbestos fibers.
3. The microporous diaphragm as defined by claim 2, at least 40% by weight
of said fibers being asbestos fibers
4. The microporous diaphragm as defined by claim 1, comprising a gel of an
oxohydroxide of at least one Group IVA and IVB metal.
5. The microporous diaphragm as defined by claim 1, comprising a gel of
titanium, zirconium or cerium oxohydroxide.
6. The microporous diaphragm as defined by claim 1, said fluoropolymer
binder comprising polytetrafluoroethylene.
7. The microporous diaphragm as defined by claim 1, comprising from 1% to
25% by weight of said oxohydroxide gel.
8. The microporous diaphragm as defined by claim 1, comprising at least 3%
by weight of said oxohydroxide gel.
9. The microporous diaphragm as defined by claim 1, said fibers comprising
fluoropolymer, zirconia, carbon, graphite or titanate fibers.
10. The microporous diaphragm as defined by claim 1, weighing from about
0.4 to 3 kg/m.sup.2.
11. An assembly adapted for incorporation in an electrolytic cell,
comprising a composite cathode component consolidated with the microporous
diaphragm as defined by claim 1.
12. The electrolytic assembly as defined by claim 11, said composite
cathode component comprising an elementary cathode having a highly porous
metal surface, said metal surface having deposited thereon a fluoropolymer
microconsolidated microporous fibrous sheet material which comprises an
effective amount of electrically conductive fibers.
13. The electrolytic assembly as defined by claim 12, said electrically
conductive fibers comprising carbon or graphite fibers monodisperse in
length.
14. The electrolytic assembly as defined by claim 12, said fluoropolymer
binder comprising polytetrafluoroethylene.
15. An electrolytic cell comprising the electrolytic assembly as defined by
claim 11.
Description
CROSS-REFERENCE TO COMPANION APPLICATION
Copending application Ser. No. 07/565385, filed concurrently herewith and
assigned to the assignee hereof.
BACKGROUND OF THE INVENTION
1. Field of the Invention:
The present invention relates to novel diaphragms comprising asbestos
fibers adapted for use in electrolytic cells, to the coupling of such
diaphragms with a cathode component of such cells and to a process for
producing such diaphragms and coupling such diaphragms with a cathode
component.
This invention especially relates to improved diaphragms produced via a wet
route, based on asbestos fibers, and adapted for use in chlorine/soda
electrolysis cells.
2. Description of the Prior Art:
Asbestos fibers have long been employed in this art as a conventional
material for producing the diaphragms used in electrolytic cells. These
diaphragms are fabricated by depositing asbestos fibers contained in an
aqueous mash onto a cathode which is permeable to the electrolytes, the
deposition operation being carried out under vacuum. Thus, French Patent
No. 2,213,805 describes preparing microporous separators by depositing a
layer of asbestos, said layer being consolidated by a fluoropolymer. The
porosity of such a layer can be better controlled by adding a pore-forming
agent according to the technique described in French Patent No. 2,229,739.
As is well known to this art, such preparation of microporous separators by
depositing, under vacuum, an aqueous mash containing fibers and a binder
presents a very great advantage, both from a technological standpoint, as
well as from an economic standpoint. However, the quality of the
separators thus produced is not fully satisfactory.
Indeed, the Faraday yield is insufficient, and this is reflected in a high
energy consumption per ton of chlorine produced. The higher the sodium
hydroxide concentration, the more the yield in question is lowered on an
industrial level. Thus, it is of the greatest importance to be able to
operate with concentrated sodium hydroxide, in order to reduce the energy
cost of the evaporation which is subsequently required to concentrate the
sodium hydroxide produced. It would, therefore, be desirable to provide an
improved diaphragm based on asbestos fibers and capable of being produced
by a wet route.
In published European Patent Application No. 132,425 cathode components
have been described, composite material produced by the coupling of an
elementary cathode including a highly porous metal surface such as a metal
grid having a mesh opening ranging from 20 .mu.m to 5 mm and of a sheet
containing fibers and a binder, the coupling and the sheet resulting from
the programmed suction under vacuum of a suspension containing essentially
electrically conductive fibers and a fluoropolymer, directly through said
elementary cathode, followed by drying, and then melting the binder. Such
composite materials that are adapted to themselves constitute the cathode
of an electrolysis cell and may be coupled with a diaphragm, it being
possible for the diaphragm to be manufactured directly by a wet route on
the composite.
Various improvements have also been made, both to the composite materials
themselves, as well as to the process for the manufacture thereof.
In published European Patent Application No. 214,066 materials are
described, containing carbon fibers exhibiting a monodisperse length
distribution, materials whose quality and properties are very appreciably
improved, and this is reflected in a much more favorable
performance/thickness relationship.
In published European Patent Application No. 296,076 electroactivated
materials are described which contain an electrocatalytic agent uniformly
distributed within their bulk mass, said agent being selected from among
Raney metals and Raney alloys from which most of the easily removable
metal(s) has (have) been removed.
The assemblage of proposed cathode components which ensure an appreciable
distribution of the current is adapted for use in an electrolytic cell
which will comprise a membrane or a diaphragm between the anode and
cathode compartments. Additional technical details are described in the
aforementioned European patent applications, hereby expressly incorporated
by reference in respect of the construction of said cathode components.
SUMMARY OF THE INVENTION
A major object of the present invention is the provision of improved
microporous separators comprising asbestos fibers by a wet route, by
vacuum suction of an aqueous mash containing asbestos fibers through a
porous support, which improved microporous separators conspicuously
ameliorate those disadvantages and drawbacks to date characterizing the
state of this art.
Briefly, the present invention features novel microporous diaphragms that
can be produced in situ by a wet route, comprising a fibrous sheet
containing asbestos fibers that have been microconsolidated by a
fluoropolymer, said sheet having been sintered and comprising:
(i) from 3% to 35% by weight of a fluoropolymer, binding the fibers;
(ii) from 1% to 50% by weight of a gel of an oxohydroxide of at least one
metal of Groups IVA, IVB, VB and VIB of the Periodic Table or of the
lanthanide and actinide series; and
(iii) from 20% to 95% by weight of fibers, at least 1% by weight of such
fibers being asbestos fibers.
The present invention also features the coupling of the above novel
diaphragms to a composite cathode component.
This invention also features a process for the production of such
diaphragms, comprising the following sequence of stages:
(a) preparing, in an essentially aqueous medium, a dispersion comprising
the fibers, the fluoropolymer-based binder in the form of particles, if
appropriate at least one precursor of an oxohydroxide of at least one of
the metals of Groups IVA, IVB, VB and VIB of the Periodic Table or of the
lanthanide and actinide series, also in particle form, and, if
appropriate, additives;
(b) depositing a sheet by programmed vacuum filtration of said dispersion
through a porous support material;
(c) removing the liquid medium and, if appropriate, drying the sheet thus
formed;
(d) sintering this sheet; and
(e) treating, if appropriate, in situ under the conditions of electrolysis,
the sintered sheet with an aqueous solution of an alkali metal hydroxide.
The porous material (support) may then constitute a composite cathode
component, and the above process then enables producing a coupling
according to the present invention.
Thus, the present invention also features a process for the preparation of
such couplings comprising the following sequence of stages:
(a) depositing a precathodic sheet by programmed vacuum filtration of a
dispersion, in an essentially aqueous medium, of fibers, of particulate
binder and, if appropriate, of additives, through an elementary cathode
which comprises a metal surface having mesh openings or perforations
ranging from 20 .mu.m to 5 mm;
(b) removing the liquid medium and, where appropriate, drying the sheet
thus formed;
(c) programmed vacuum filtering, through the precathodic sheet, of a
dispersion, in an essentially aqueous medium, of fibers, of particulate
binder based on a fluoropolymer, of at least one precursor of an
oxohydroxide of at least one of the metals of Groups IVA, IVB, VB and VIB
of the Periodic Table or of the lanthanide and actinide series, in the
form of particles, and, if appropriate, of additives;
(d) removing the liquid medium and, where appropriate, drying the sheet
thus formed;
(e) sintering of such sheet; and
(f) treating, if appropriate in situ under the conditions of the
electrolysis, such sintered sheet with an aqueous solution of an alkali
metal hydroxide.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
More particularly according to the present invention, the subject novel
diaphragms exhibit an appreciable dimensional stability, a fine and
uniform porosity and a permanent wettability. The diaphragms according to
the present invention additionally exhibit very low operating voltages,
and this constitutes another advantage of this invention.
The diaphragms according to the present invention are advantageously
produced by the routes, traditionally employed industrially, of deposition
of a suspension by suction under vacuum, and permit the efficient (high
current yield) operation of brine electrolysis cells comprising same, at
high current densities which can extend up to 40 A/dm.sup.2 and higher.
Moreover, such diaphragms permit cell operation at high sodium hydroxide
concentrations (on the order of 140 to 200 g/l or higher) in the
catholyte, and this limits the energy consumption required for the
subsequent concentration of sodium hydroxide.
The diaphragms according to the invention comprise an asbestos-based
fibrous sheet. By "sheet" is intended a three-dimensional assembly or
stack whose thickness is appreciably smaller than its other dimensions, it
being possible for said assembly, if appropriate, to have two parallel
face surfaces. These sheets can be in various forms, generally determined
by the geometry of the cathode components with which they may later be
coupled. In their use as microporous diaphragms in cells for the
electrolysis of sodium chloride, and by way of example, their thickness
typically ranges from 0.1 to 5 mm, and one of their long dimensions,
corresponding substantially to the height of the cathode component, can
extend up to 1 m or even more, and the other long dimension, reflecting
substantially the perimeter of the component in question, can extend up to
several tens of meters.
The fibers of the sheet are microconsolidated, namely, they are, in a
fashion, attached or interconnected to each other, above all, by a
three-dimensional network of discrete bonding points, and this contributes
to ensuring that the sheet has a porosity which is at the same time fine
and uniform, and a very great cohesion.
These sheets (or fibrous stacks) according to the invention are based on
asbestos and essentially consist of, as indicated above:
(i) from 3% to 35% by weight of a fluoropolymer binder;
(ii) from 1% to 50% by weight of a gel of an oxohydroxide of at least one
metal of Groups IVA, IVB, VB and VIB of the Periodic Table or of the
lanthanide and actinide series; and
(iii) from 20% to 95% by weight of fibers, at least 1% by weight of said
fibers being asbestos fibers.
By "fluoropolymer" is intended a homopolymer or a copolymer derived at
least partly from olefinic monomers completely substituted by fluorine
atoms or completely substituted by a combination of fluorine atoms and of
at least one of chlorine, bromine or iodine atoms per monomer.
Exemplary fluoro homo- or copolymers include polymers and copolymers
derived from tetrafluoroethylene, hexafluoropropylene,
chlorotrifluoroethylene and bromotrifluoroethylene.
Such fluoropolymers may also contain up to 75 mole percent of recurring
structural units derived from other ethylenically unsaturated monomers
containing at least as many fluorine atoms as carbon atoms, such as, for
example, vinylidene (di)fluoride, or vinyl perfluoroalkyl ethers such as
perfluoroalkoxyethylene.
A plurality of fluoro homo- or copolymers such as described above can, of
course, be used according to the present invention. It, too, will be
appreciated that it is also within the scope of this invention to use
together with these fluoropolymers a small quantity, for example up to 10%
or 15% by weight of polymers that do not contain fluorine atoms, such as,
for example, polypropylene.
Polytetrafluoroethylene is the preferred binder of the diaphragms according
to the invention.
The fluoropolymer employed as the binder for the assembly of fibers may be
present in the subject diaphragms in amounts which can vary over wide
limits, bearing in mind the fiber content and the nature of the various
constituents of such diaphragms.
However, to ensure a good consolidation of the assembly, the binder will
preferably constitute from 5% to 40% by weight in the subassembly
(fibers+binder).
The diaphragms according to the invention also comprise from 20% to 95% by
weight of fibers.
These fibers, of which at least 1% (by weight) and, preferably at least 40%
(by weight) are asbestos fibers, can be of various types. It is possible,
indeed, to use different inorganic fibers, organic fibers, or mixtures of
inorganic fibers and organic fibers. Exemplary organic fibers for forming
the diaphragms according to the present invention include fibers of the
fluoropolymers indicated above and, more particularly,
polytetrafluoroethylene (PTFE) fibers.
The PTFE fibers according to the present invention may be of variable
sizes; their diameter (D) typically ranges from 10 to 500 .mu.m and their
length (L) is such that the ratio (L/D) advantageously ranges from 5 to
500. Those preferred are PTFE fibers having mean dimensions ranging from 1
to 4 mm in the case of length and ranging from 50 to 200 .mu.m in the case
of diameter. Their preparation is described in U.S. Pat. No. 4,444,640 and
PTFE fibers of this type are known to the art.
Exemplary inorganic fibers for forming the diaphragms according to the
present invention include zirconia, carbon, graphite or titanate fibers.
The carbon or graphite fibers are in the form of filaments whose diameter
is generally smaller than 1 mm and preferably ranges from 10.sup.-5 to 0.1
mm and whose length is greater than 0.5 mm and preferably ranges from 1 to
20 mm.
These carbon or graphite fibers preferably have a monodisperse length
distribution, namely, a distribution of lengths such that the length of at
least 80%, and advantageously of at least 90%, of the fibers is within
.+-.20%, and preferably within .+-.10%, of the average length of the
fibers. When they are present, these carbon fibers advantageously
constitute not more than 10% by weight of the combined fibers.
The titanate fibers are fibrous materials which are known per se. Thus,
potassium titanate fibers are available commercially. Other suitable
fibers are those derived from potassium octatitanate K.sub.2 Ti.sub.8
O.sub.17 by partial replacement of the ions of titanium in the oxidation
state of IV with metal cations in the oxidation state of II, such as
magnesium and nickel cations, or in the oxidation state of III, such as
iron or chromium cations, and displaying charge compensation ensured by
alkali metal ions such as sodium and potassium cations. These are
described in published French Patent Application No. 2,555,207.
Other titanate fibers, such as those of potassium tetratitanate (K.sub.2
Ti.sub.4 O.sub.9) or those derived therefrom, can also be employed. While
the titanate fibers can, without major inconvenience, constitute up to 80%
by weight of the fiber mixture employed, it is preferable, when using
carbon or graphite fibers, that their proportion in the fiber mixture
should not exceed 10% by weight.
Mixtures of inorganic fibers which differ in their nature can, of course,
also be employed.
The diaphragms according to the invention also contain from 1% to 50% by
weight of a gel of an oxohydroxide of at least one metal of Groups IVA,
IVB, VB and VIB of the Periodic Table or of the lanthanide and actinide
series. The gel content preferably constitutes from 2% to 25% by weight
and, more preferably, at least 3% by weight.
This gel is distributed uniformly both at the surface of the diaphragms
according to the invention and into the depth thereof.
The content of gel, initially impregnated with sodium chloride, sodium
hydroxide and water, is determined after contact at 85.degree. C. with an
aqueous solution containing 140 g/l of sodium hydroxide and 160 g/l of
sodium chloride, followed by cooling to 25.degree. C., washing with water
and drying for 24 hours at 100.degree. C.
Examples of the metals of the Groups and series of the Periodic Table which
are noted above which are representative are titanium, zirconium, thorium,
cerium, tin, tantalum, niobium, uranium, chromium and iron. Mixtures of
such metals or of such metals and of alkali metals such as sodium or
potassium may, of course, be present in the diaphragms according to the
invention.
The diaphragms according to the invention preferably contain a gel of an
oxohydroxide of at least one metal of Groups IVA and IVB.
They preferably contain a titanium, zirconium or cerium oxohydroxide gel.
The diaphragms according to this invention have been defined in terms of
their essential constituents. It will be appreciated that these materials
can contain various other additives in a minor amount generally not
exceeding 5% by weight, which will have been added either simultaneously
or successively during any of the stages of their production. Thus, they
can contain trace amounts of surface-active or surfactant agents, of
pore-forming agents whose function is to regulate the porosity of the
diaphragm, and/or of thickeners, although such agents are in principle
decomposed or removed during the production of the said diaphragm.
The diaphragms according to the present invention advantageously have a
weight per unit of surface area ranging from 0.4 to 3 kg/m.sup.2 and
preferably from 0.7 to 1.9 kg/m.sup.2.
The present invention also features the coupling of a composite cathode
component and of a diaphragm as described above.
These composite cathode (or precathodic) components are produced by the
coupling of an elementary cathode comprising a highly porous metal surface
and a microporous fibrous sheet containing a significant proportion of
electrically conductive fibers, such fibers being microconsolidated by a
fluoropolymer.
The preferred cathode (or precathodic) components according to the present
invention contain carbon or graphite fibers as electrically conductive
fibers. These fibers preferably have a monodisperse length distribution.
Although the fluoropolymer binder for the precathodic sheet may be selected
from the fluoropolymers described above, polytetrafluoroethylene is the
preferred.
These composite cathode (or precathodic) components are described in the
European patent applications hereinbefore incorporated by reference.
It will be apparent that the coupling under consideration entails an
assembly of one face surface towards the other of three layers, namely,
the elementary cathode, a first fibrous sheet containing the electrically
conductive fibers, such sheet having intrinsic properties which are
described in said European patent applications, and the diaphragm, said
assembly constituting a coherent complete organization.
Also as indicated above, the present invention also features a process for
the production of the diaphragms described immediately above.
The process for the preparation of the subject diaphragms comprises the
following sequence of stages:
(a) preparing, in an essentially aqueous medium, a dispersion comprising
the fibers, the fluoropolymer-based binder in the form of particles, at
least one precursor of an oxohydroxide of at least one of the metals of
Groups IVA, IVB, VB and VIB of the Periodic Table or of the lanthanide and
actinide series, in the form of particles and, if appropriate, additives;
(b) depositing a sheet by programmed vacuum filtration of said dispersion
through a porous support material;
(c) removing the liquid medium and, if appropriate, drying the sheet thus
formed;
(d) sintering such sheet; and
(e) treating, if appropriate, in situ under the conditions of the
electrolysis, such sintered sheet with an aqueous solution of alkali metal
hydroxide.
By "essentially aqueous medium" is intended a medium containing no organic
compounds other than the constituents indicated above and additives such
as surface-active agents, surfactants and thickeners. Thus, the medium in
question does not contain any organic solvent.
Indeed, while the presence of organic solvents is not detrimental in and of
itself, one advantage presented both by the present process and the
diaphragms according to the invention is the fact that the presence of
organic solvents is not necessary for the production of such diaphragms
and that, as a result, it is not necessary to provide an additional stage
of evaporation of the solvent.
By "precursors of an oxohydride of one of the metals of Groups IVA, IVB, VB
and VIB of the Periodic Table or of the lanthanide and actinide series"
are intended salts of said metals, as poorly soluble as possible in water,
in which the anion is advantageously selected from among the phosphate,
pyrophosphate, hydrogen phosphate or polyphosphate anions, substituted
where appropriate by an alkali metal, and silicate.
Exemplary salts which can be used according to the present process include:
Titanium phosphate (.alpha.-TiP)
Zirconium phosphate (.alpha.-ZrP)
Cerium phosphate
Ti(NaPO.sub.4).sub.2
TiNaH(PO.sub.4).sub.2
TiP.sub.2 O.sub.7
TaH (PO.sub.4).sub.2
NbOPO.sub.4
UO.sub.2 HPO.sub.4
Cr.sub.5 (P.sub.3 O.sub.10).sub.3
Fe.sub.4 (P.sub.2 O.sub.7).sub.3
The compounds corresponding to the formula
M.sub.1+x Zr.sub.2 SixP.sub.3-x O.sub.12
in which M is a sodium or lithium atom and x is a number which may be zero
and smaller than 3.
These precursors are introduced in the form of particles. They may be
introduced in the form of a powder having a particle size which is
generally smaller than 500 .mu.m or in the form of fibers whose dimensions
typically range from 0.1 to 50 .mu.m in the case of diameter and from 3
.mu.m to 3 mm in the case of length.
The fluoropolymer-based binder is typically in the form of a dry powder or
of an aqueous dispersion (latex) whose solids content constitutes from 30%
to 80% by weight.
As is well known to this art, the dispersion or suspension under
consideration is generally highly dilute, the content of dry matter
(fibers, binder, precursors and additives) representing on the order of 1%
to 15% of the weight of the entire mass, to make it easier to handle on an
industrial scale.
Various additives may also be introduced into the dispersion, in particular
surface-active agents or surfactants such as octoxynol (Triton
X-100.RTM.), pore-forming agents such as silica, and thickening agents
such as natural or synthetic polysaccharides.
The dispersion will obviously contain all of the essential constituents of
the diaphragm with the exception of the oxohydroxide gel discussed above,
but gel precursors as described above will be present.
The relative amounts of the essential constituents of the diaphragm to be
introduced into the dispersion can be readily determined by one skilled in
this art, in consideration of the fact that they are substantially the
same as those present in the diaphragm itself, with the exceptions of the
pore-former which is in principle removed by the action, for example, of
the electrolytic sodium hydroxide and of the oxohydroxide gel precursor.
Indeed, the precursor is completely converted into oxohydroxide gel in
which the "active" part obtained after washing and drying the gel,
constitutes from 10% to 90% by weight of the precursor introduced.
One skilled in this art can also easily determine, using simple tests, the
amount of dry solids to be dispersed in the aqueous medium as a function
of the degree of retention which can be observed on the porous material
through which the dispersion is filtered under the programmed vacuum
conditions.
In general, the solids content in suspension comprises, as the principal
constituents:
(i) from 30% to 80% by weight of fibers;
(ii) from 1% to 50% by weight of at least one oxohydroxide gel precursor;
(iii) from 5% to 35% by weight of PTFE powder (binder); and
(iv) from 5% to 40% by weight of silica.
To satisfactorily carry out the present invention, the content of PTFE
powder constitutes from 5% to 40% by weight of the entire mass (PTFE
powder+fibers). Also to advantageously carry out the present invention,
the weight content of at the least one oxohydroxide gel precursor in said
solids content will range from 5% to 40%.
The sheet is then formed by programmed vacuum filtration of the dispersion
through a porous material such as cloths or grids in which the mesh
opening, the perforations or the porosity, advantageously ranges from 1
.mu.m to 2 mm.
The vacuum program may be continuous or stagewise, from atmospheric
pressure to the final pressure (0.01 to 0.5 bars absolute).
After removal of the liquid medium and, where appropriate, the drying of
the sheet thus obtained, the sheet is sintered.
The sintering is carried out in a manner known per se at a temperature
above the melting or softening point of the fluoropolymer binding the
sheet. This stage, which permits the sheet to be consolidated, is then
followed by a stage of treatment, by means of which the sheet is contacted
with an aqueous solution of alkali metal hydroxide, and more particularly
with an electrolytic sodium hydroxide solution.
This contacting may be conducted in situ, namely, when the consolidated
sheet is placed in the electrolytic cell, in contact with the electrolytic
sodium hydroxide solution.
The contacting is advantageously carried out with an aqueous solution of
sodium hydroxide, the concentration of which ranges from 40% to 200 g/l
and at a temperature ranging from 20.degree. to 95.degree. C.
The precursors of the oxohydroxide gel, described above, are capable of
undergoing various conversions during the various operations of production
of the diaphragm, and especially a nondestructive conversion during the
sintering operation, i.e., resulting solely in losses of molecules of
water of hydration or of formation; they will be converted by the
aforementioned treatment stage into a fresh gel of oxohydroxide of the
metal in question, impregnated with electrolyte and with water.
The properties of a diaphragm of this type are markedly improved.
Moreover, using precursors in the form of powder renders the processing
considerably easier.
In a preferred alternate embodiment of the invention, the filtration of the
dispersion or suspension is carried out through a cathode (or precathodic)
component as defined above.
Such alternate embodiment enables production of a diaphragm/precathodic
component coupling.
Such a coupling exhibits remarkable coherence properties, linking together
the advantages presented by the precathodic component and the diaphragms
according to the invention.
This invention also features a process for the preparation of such
couplings, comprising the following sequence of stages:
(a) depositing a precathodic sheet by programmed vacuum filtration of a
dispersion, in an essentially aqueous medium, of fibers, of binder in the
form of particles and, if appropriate, of additives, through an elementary
cathode which comprises a metal surface exhibiting a mesh opening or
perforations ranging from 20 .mu.m to 5 mm;
(b) removing the liquid medium and, where appropriate, drying the sheet
thus formed;
(c) programmed vacuum filtering, through the precathodic sheet, of a
dispersion, in an essentially aqueous medium, of fibers, of binder based
on a fluoropolymer in the form of particles, of at least one precursor of
an oxohydroxide of at least one of the metals of Groups IVA, IVB, VB and
VIB of the Periodic Table or of the lanthanide and actinide series, in the
form of particles and, where appropriate, of additives;
(d) removing the liquid medium and, where appropriate, drying the sheet
thus formed;
(e) sintering of the resulting sheet; and
(f) treating, if appropriate in situ under the conditions of the
electrolysis, the sintered sheet with an aqueous solution of alkali metal
hydroxide.
Such a process presents the advantage of contributing to establishing
couplings of great cohesion. Another advantage is its great simplicity of
implementation due to the fact that a single sintering stage is sufficient
to produce couplings of high cohesion and due to the fact that a single
stage makes it possible to remove the pore-formers, both from the
precathodic sheet and from the diaphragm, and to provide fresh gel of
oxohydroxide of the metal under consideration.
In another preferred alternate embodiment of such process, PTFE is used as
a binder for the precathodic sheet and the diaphragm.
In order to further illustrate the present invention and the advantages
thereof, the following specific examples are given, it being understood
that same are intended only as illustrative and in nowise limitative.
EXAMPLES
A suspension of the following constituents was prepared with stirring:
(A) softened water, the amount of which was calculated to provide
approximately 4 liters of suspension;
(B) 100 g of chrysotile asbestos fibers, 200 angstroms in diameter and at
least 1 mm in length;
(C) 1.2 g of octoxynol in the form of a solution in water at a
concentration of 40 g/l.
Stirring was carried out for 30 min and then the following various
constituents were added in succession, with stirring:
(D) 25 g of PTFE in the form of a latex containing approximately 65% by
weight of solids;
(E) 30 g of precipitated silica in the form of particles having a mean
particle size of 3 .mu.m and whose BET surface area was 250 m.sup.2 /g;
(F) if appropriate, X g of titanium phosphate (.alpha.-TiP), zirconium
phosphate (.alpha.-ZrP) or cerium phosphate (CeP) powder;
(G) 1.5 g of xanthan gum.
Stirring was carried out for 30 min.
The total volume of water was calculated such that the weight percentage of
dry solids (B+D+E+F)/A was approximately 4.5%.
The solution was maintained for 48 hours.
The required volume of solution was withdrawn, such that it contained the
amount of solids content intended to be deposited to form the diaphragm
(on the order of 1.3 kg/m.sup.2).
The suspension was stirred again for 30 min before use.
The filtration was conducted under a programmed vacuum on a bulk cathode
(prepared beforehand according to Example 7 of European Patent Application
No. 296,076) as follows:
1 min at a vacuum of -5 to -10 mbar relative pressure in relation to
atmospheric pressure;
increasing the vacuum at a rate of 50 mbar/min;
dewatering for 15 min at maximum vacuum (approximately -800 mbar relative
pressure in relation to atmospheric pressure)
The composite was then sintered after optional drying at 100.degree. C.
and/or intermediate stabilization of the temperature, by heating the
cathode assembly and the diaphragm to 350.degree. C. for 7 min.
The performances of the various composite materials, the production of
which was as described immediately above, were then evaluated in an
electrolysis cell which exhibited the following characteristics and the
operating conditions of which were as indicated below:
Rolled, expanded titanium anode coated with TiO.sub.2 -RuO.sub.2 ;
Cathode component made of braided and rolled mild steel; 2 mm wires, 2 mm
mesh covered with the precathodic sheet and the diaphragm;
Anode/cathode component distance: 6 mm;
Active surface area of the electrolyzer: 0.5 dm.sup.2 ;
Cell assembled according to the filter press type;
Current density: 25 A dm.sup.-2
Temperature: 85.degree. C.
Operation at constant anode chloride: 4.8 mol 1.sup.-1 ;
Electrolytic sodium hydroxide concentration 120 or 200 g/l.
The particular conditions and the results obtained are reported in the
Table below:
FY: Faraday yield
.DELTA.U: voltage at the terminals of the electrolyzer under the specified
current density.
Performance (kW h/T Cl.sub.2)=energy consumption of the system, in kilowatt
hours per ton of chlorine produced.
TABLE:
__________________________________________________________________________
Weight NaOH
Example
Phosphate
Quantity
deposited
.DELTA.U
FY Concentration
Performance
No. type X (g)
1 g/m.sup.2
Volts
(%)
g/l kW h/T Cl.sup.2
__________________________________________________________________________
1 -- 0 1.25 3.13
96.5
120 2,450
85 200 2,785
2 .alpha.-TiP
10 1.31 3.18
88.5
200 2,715
3 .alpha.-TiP
25 1.30 3.13
98.5
120 2,400
90 200 2,630
4 .alpha.-TiP
35 1.21 3.35
91 200 2,785
5 .alpha.-ZrP
15 1.25 3.08
86.5
200 2,690
6 CeP 15 1.34 3.20
90 200 2,690
__________________________________________________________________________
While the invention has been described in terms of various preferred
embodiments, the skilled artisan will appreciate that various
modifications, substitutions, omissions, and changes may be made without
departing from the spirit thereof Accordingly, it is intended that the
scope of the present invention be limited solely by the scope of the
following claims, including equivalents thereof.
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